1. Field of the Invention
The present invention relates to the detection of contaminants, and more specifically, it relates to a unit which performs contamination analysis.
2. Description of Related Art
One of the biggest sources of hazardous waste and VOC air emissions throughout American industry is parts and equipment cleaning operations. These operations include cleaning related to metal fabrication and finishing processes such as machining and electroplating, as well as electronic fabrication processes that include printed circuit board manufacture and component assembly activities. Parts and equipment cleaning is an integral part of a wide range of major industries such as aerospace, electronics equipment and computer manufacture, medical equipment manufacture, chemical manufacturing, and many others.
In all of these industry segments, large quantities of hazardous solvents (both halogenated and nonhalogenated) are routinely employed, and eventually find their way into a waste stream, or are emitted into the air. In 1991 for instance, the U.S. demand for four commonly used halogenated cleaning solvents (trichloroethylene, perchloroethylene, methylene chloride, and 1,1,1 trichloroethane) totaled nearly 200,000 metric tons; of this, one gallon in three was used for parts and equipment cleaning.
A common, and effective approach to hazardous cleaning solvent waste and emissions reduction has been to substitute environmentally more benign materials whenever possible. While this is an excellent approach and is responsible for significant pollution prevention, there are still many cleaning applications in which chlorinated or other hazardous and volatile solvents are required. In these cases, it is essential to use the chemicals in as efficient and conservative a manner as possible. Waste due to unnecessary parts cleaning, or due to recleaning parts that were improperly handled the first time, should be avoided through accurate process controls and contamination analysis procedures.
Unfortunately, this is not the case in most industries. While many U.S. manufacturing processes are now state of the art and highly efficient, parts and equipment cleaning lags sorely behind, especially in the area of real-time process controls. One of the most common methods in aircraft manufacture, for instance, for determining when a wing or fuselage section is clean enough, is a water-break test. This highly variable, non-quantitative approach determines that a part is supposedly clean when water runs off the surface in a sheet rather than beading up. This method has been referred to as a "nineteenth century approach", and varies markedly depending on the cleaners used (for instance, detergent on a part surface will cause water to sheet off and make the surface appear clean, even when considerable soil can be present). Laboratory analyses of surface contamination are also used as spot checks of cleaning performance, but these tests have turnaround times of several days or longer, and often don't identify a problem until many parts have been improperly cleaned.
A need exists for a real-time feedback mechanism for verification of cleaning performance in the aerospace industry. Electronics companies also have many applications for such technology. The method of minimizing cleaning problems in some instances has been to use procedures that overclean most parts in the hopes of adequately cleaning all of them. Improper cleaning is sometimes not detected until after an assembly is completely built.
A need exists for a sensing technology that can provide real-time cleaning verification feedback in an industrial production line environment, and to make this technology robust enough that it can be used in a wide range of industries and particular applications. The technology should be portable so that it can quickly be moved from one part of an assembly line to another. It should generate highly precise data. Hydrocarbon contamination layer thicknesses of a fraction of nanometer should be routinely measurable. This corresponds to contamination one or two atomic layers thick that can be measured. The sensor should identify the type of contamination, distinguish between different hydrocarbon species, and detect other common contaminants, such as silicone oils. Finally, the components of the sensor should be inexpensive. The present invention provides these advantages.